The invention relates to a process for recovery of natural gas liquid (NGL) from pressurized liquid natural gas (PLNG).
Because of its clean burning qualities and convenience, natural gas has become widely used in recent years. Many sources of natural gas are located in remote areas, great distances from any commercial markets for the gas. Sometimes a pipeline is available for transporting produced natural gas to a commercial market. When pipeline transportation is not feasible, produced natural gas is often processed into liquefied natural gas (which is called xe2x80x9cLNGxe2x80x9d) for transport to market.
It has been proposed to transport natural gas at temperatures above xe2x88x92112xc2x0 C. (xe2x88x92170xc2x0 F.) and at pressures sufficient for the liquid to be at or below its bubble point temperature. This pressurized liquid natural gas is referred to as xe2x80x9cPLNGxe2x80x9d to distinguish it from LNG, which is transported at near atmospheric pressure and at a temperature of about xe2x88x92162xc2x0 C. (xe2x88x92260xc2x0 F.). The term xe2x80x9cbubble pointxe2x80x9d means the temperature and pressure at which a liquid begins to convert to gas. For example, if a certain volume of PLNG is held at constant pressure, but its temperature is increased, the temperature at which bubbles of gas begin to form in the PLNG is the bubble point. Similarly, if a certain volume of PLNG is held at constant temperature but the pressure is reduced, the pressure at which gas begins to form defines the bubble point pressure at that temperature. At the bubble point, the liquefied gas is saturated liquid. For most natural gas compositions, the bubble point pressure of the natural gas at temperatures above xe2x88x92112xc2x0 C. will be between about 1,380 kPa (200 psia) and about 4,480 kPa (650 psia). Processes for making PLNG are disclosed in U.S. Pat. No. 5,950,453 by R. R. Bowen et al.; U.S. Pat. No. 5,956,971 by E. T. Cole et al.; U.S. Pat. No. 6,016,665 by E. T. Cole et al.; and U.S. Pat. No. 6,023,942 by E. R. Thomas et al.
In conventional LNG plants, essentially all pentanes and heavier hydrocarbons (C5+) are typically removed to avoid potential freezing problems at its usual storage temperatures of about xe2x88x92162xc2x0 C. (xe2x88x92260xc2x0 F.). One of the advantages of producing and shipping PLNG at a warmer temperature is that PLNG can contain considerably more C5+ components than can be tolerated in most LNG applications.
Depending upon market prices for ethane, propane, butanes, and the heavier hydrocarbons (collectively referred to herein as xe2x80x9cNGL productsxe2x80x9d), it may be economically desirable to recover the NGL products from the PLNG and to sell them as separate products. A need therefore exists for a process for effectively removing NGL products from PLNG.
The invention is a process for removal of hydrocarbons less volatile than methane from a pressurized liquid natural gas (PLNG). In this process PLNG is heated in a heat exchanger, thereby vaporizing at least a portion of the PLNG. The partially vaporized PLNG is passed to a fractionation column. A liquid stream enriched with hydrocarbons (C2+ or C3+) less volatile than methane is withdrawn from a lower portion of the fractionation column and a vapor stream lean in the hydrocarbons less volatile than methane is withdrawn from an upper portion of the fractionation column. The withdrawn vapor stream is passed to the heat exchanger to condense the vapor to produce PLNG lean in hydrocarbons less volatile than methane.